Waveguide filter with three apertures for passing transmission frequencies and blocking interference frequencies

ABSTRACT

A band-pass filter for a waveguide, which additionally serves as a blocking filter for the concerted suppression of interference frequencies. In addition to a centrally arranged aperture ( 3 ), a waveguide aperture has two identically designed apertures ( 4 ), which are laid out for natural resonance for an interference frequency f (ind S 1 ) to be blocked by the waveguide filter, and are arranged symmetrical to the central aperture opening ( 3 ). In waveguide devices, the invention is used to meet the statutory frequency specifications and simultaneously suppress interference frequencies.

BACKGROUND OF THE INVENTION

The invention concerns a waveguide filter of the type that is insertedas a planar conducting structure in a cross-sectional plane of anelongated waveguide, perpendicular to the waveguide axis, between afeeding and an extending segment of the waveguide, and comprising atleast one aperture configured for natural resonance for a selectedtransmission frequency.

Meeting the statutory regulations for the frequency band specifications,particularly with respect to the suppression of oscillator-generatedinterference radiation, e.g., a harmonic of the useful signal,represents a considerable share of the development costs for anequipment concept. At present, the use of waveguide resonators coupledto waveguide apertures as well as the use of resonance structures forFIN line circuits in the region of the fin-lines are known for waveguidecircuits.

The layout of waveguide apertures is based on the known theories inwaveguide aperture technology (Waveguide Handbook, N. Marcuwitz, McGraw-Hill Book Company, INC., Edition 1986). The waveguide apertures areformed as planar structures, for example with circular, slotted orH-shaped apertures and are inserted into a cross-sectional plane of anelongated waveguide, perpendicular to the waveguide axis, between afeeding and an extending segment of the waveguide. When used asband-pass filters, the apertures are operated with natural resonance,which for slotted apertures is set to an electrically effective slotlength of one half the wavelength for the useful frequency, as is known.The transmission factor T of slotted apertures is determined by the slotwidth. Outside of their pass range, the waveguide apertures designed asband-pass filters have an attenuation course that corresponds only tothe resonance curve. In particular, no concerted blocking of specificinterference frequency multiples of the useful frequency is generallypossible with these band-pass filters. For this, the known band-passfilters must be supported by additional filtering measures. In manycases, e.g. with small devices or with sensors, the volume required forthe known band-pass filters causes interferece. The production costs forFIN conductors with resonance structures are relatively high.

A waveguide filter is furthermore disclosed in the EP 0 029 276 A1,which is composed of four planar conducting structures that cooperate inthe waveguide field and which are integrated into the wall of awaveguide that is fed by a transmitter. The conducting structures areintegrated across the circumference of the waveguide in across-sectional plane into the wall area, displaced by 90° to eachother, and respectively feed into an extending waveguide segment. Inaddition to a central aperture that is laid out for natural resonancefor a transmission frequency, each of the conductive structures has twoidentically configured apertures, designed to suppress interferencefrequencies. These apertures are arranged on one longitudinal side ofthe central aperture, so that with respect to their longitudinalexpansion, they are not positioned in the center of the symmetry axisfor the conducting structure, which runs parallel to the field strengthvectors.

It is the object of the invention to create a band-pass filter forwaveguides, which simultaneously functions as a blocking filter for thetargeted suppression of interference frequencies.

SUMMARY OF THE INVENTION

The above object is solved in accordance with a first aspect of theinvention a waveguide filter that is inserted as a planar conductingstructure in a cross-sectional plane of an elongated waveguide,perpendicular to the waveguide axis, between a feeding and an extendingsegment of the waveguide, that comprises a centrally arranged apertureformed for natural resonance for a selected transmission frequency f(ind 0), and that has, in addition to the centrally arranged aperture,two identically configured, parallel, spaced-apart apertures, andwherein: the identically configured apertures are formed for naturalresonance for an interference frequency f (ind S1) to be blocked by thewaveguide filter, and the apertures are arranged in the conductingstructure such that, relative to a longitudinal expansion of therespective apertures, the apertures are positioned respectively in thecenter of a symmetry axis for the conducting structure, which axis runsparallel to the field strength vectors for the waveguide.

The above object is achieved according to a further aspect of theinvention by a waveguide filter, that is inserted as a planar conductingstructure in a cross-sectional plane of an elongated waveguide,perpendicular to the waveguide axis, between a feeding and an extendingsegment of the waveguide, and comprising at least one apertureconfigured for natural resonance for a selected transmission frequency f(ind 0), and wherein the conducting structure has a centrally arrangedaperture configured for natural resonance for an interference frequencyf (ind S1) to be blocked with the waveguide filter; and, in addition tothe centrally arranged aperture, the conducting structure comprises twoidentically designed apertures, which are configured for naturalresonance for a selected transmission frequency f (ind 0) and arearranged symmetrical to the central aperture. Modifications of theinvention are described.

The advantage of this invention is that the transmission of a usefulfrequency and a targeted suppression of interference frequencies arepossible with a single component and at low expenditure. The waveguidefilter according to the invention makes it possible to adhere toproduction tolerances at a low cost and requires only a low installationdepth. Owing to its symmetrical arrangement, the excitation of theinterference mode is at a minimum.

Exemplary embodiments of the invention are explained in more detail withthe aid of the drawing.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the principle of the conductive structure of a waveguidefilter according to the invention for rectangular waveguides.

FIGS. 2A to 2C show transmission curves of individual apertures and forthe complete waveguide filter.

FIG. 3 shows a double-circuit waveguide filter with two successivelyarranged conductive structures.

FIGS. 4A to 4D show transmission curves of individual apertures and ofthe complete double-circuit waveguide filter.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows the conductive structure 2 of a waveguide filter, which isplaced onto the cross-sectional surface of a rectangular waveguide 1.The conductive structure 2 is formed in a metal layer of a planar,dielectric substrate that is metallized on one side. A centrallyarranged aperture 3 and two additional apertures 4, arrangedsymmetrically adjacent to the centrally positioned aperture 3, areintegrated into the conductive structure 2.

The apertures 3, 4 are designed as slotted guides, short-circuited onboth sides, and are arranged parallel to each other and perpendicular tothe vectors 5 of the adjoining waveguide-E-field. The electricallyeffective length of the centrally arranged slotted guide is half awavelength of the transmission frequency f (ind 0), and the electricallyeffective length of the associated slotted guide respectively is half awavelength of an interference or harmonic frequency f (ind S1) to beblocked by the waveguide filter. The slotted guides dimensioned in thisway are in natural resonance for the coordinated frequencies and radiatewith maximum capacity into the adjoining, extending segment of thewaveguide 1. The transmission phases of the centrally arranged slottedguide and those of the coordinated slotted guides have opposite signs inthe frequency range f (ind 0)-f (ind S1) and a differing phase slope, sothat in the extending segment of waveguide 1, the amount for the sumvector of the superimposing slotted guide fields is minimized. It isknown that the transmission factor (and thus also the amplitude of theE-field radiated from the slotted guide) can be influenced by varyingthe electrically effective slotted guide width. An optimum compensationof the radiated slotted guide fields can be adjusted with this in theextending segment of the waveguide 1 by adapting the individual slotwidths relative to each other for the interference frequency f (ind S1)to be blocked.

The effect according to the invention can also be achieved optionally ifonly one slotted guide 4 is assigned to the centrally arranged slottedguide 3 in the manner as described in the above. However, assigning twoslotted guides 4 has the advantage that the excitation of theinterference modes is at a minimum owing to the symmetrical structure ofthe waveguide filter. Arranging the two coordinated slotted guides 4near the border 6 of the waveguide inside space also contributes tothis.

On principal, it is possible to invert the assignment of thetransmission frequency and the interference frequency to be blocked withthe apertures 3 and 4, so that the centrally arranged aperture 3 isconfigured for the interference frequency f (ind S1) and the coordinatedapertures 4 for the transmission frequency f (ind 0).

In place of the exemplary embodiment with slotted guides, shown in FIG.1, it is also conceivable to use other cross-sectional shapes havingnatural resonances for the desired frequencies for the apertures 3 and4, e.g., H-shaped apertures.

FIGS. 2A to 2C show the transmission curves for the waveguide filteraccording to FIG. 1. The transmission curve for the centrally arrangedslotted guide 3 is shown in FIG. 2A the transmission curve for thecooperating, coordinated slotted guides 4 is shown in FIG. 2B and thetransmission curve for all superimposed slotted guide fields of thewaveguide filter is shown in FIG. 2C.

A waveguide filter with two circuits for the blocking effect can berealized in accordance with the embodiment shown in FIG. 3 in that twospaced-apart conductor structures 2, for which the design basicallycorresponds to the embodiment in FIG. 1, are inserted into thewaveguide.

The distance between the conducting structures 2 is one fourth thewavelength of the transmission frequency f (ind 0). The naturalresonances for the coordinated slotted guides of the one conductingstructure 2 are laid out for a first interference frequency f (ind S1)and the natural resonances of the second conducting structure for asecond interference frequency f (ind S2). In both conducting structures,the natural resonances of the centrally arranged slotted guides 3 arelaid out for the transmission frequency f (ind 0).

FIGS. 4A to 4D show the transmission curves for the waveguide filtershown in FIG. 3. FIG. 4A shows the transmission curve for one of thecentrally arranged slotted guides. The FIGS. 4B and 4C show thetransmission curves for the two cooperating, coordinated slotted guidesof a conductive structure for the respective interference frequency.FIG. 4D shows the transmission curve for the complete, two-circuitwaveguide filter. In addition to the blocking effects for theinterference frequencies f (ind S1) and f (ind S2), the curve shows inthe transmission range a flattening around the frequency f (ind 0).

The waveguide filter according to the invention can also be configuredfor waveguides with different cross-sectional shapes, wherein attentionmust be paid to a perpendicular alignment of the slotted guides relativeto the vectors 5 of the adjoining waveguide-E-field.

The conducting structure 2 can be a metal foil with a thickness not toexceed one eighth the wavelength of the highest interference frequencyto be blocked. However, it can also be formed in the metal layer of adielectric substrate that is metallized on one side, wherein thefiltering qualities worsen with an increasing value for the dielectricconstant of the substrate.

What is claimed is:
 1. A waveguide filter inserted as a planar firstconducting structure in a cross-sectional plane of an elongatedwaveguide, perpendicular to the waveguide axis, between a feeding and anextending segment of the waveguide, with the first conducting structurehaving a centrally arranged aperture formed for natural resonance for aselected transmission frequency f (ind 0), and, in addition to thecentrally arranged aperture (3), two identically configured, parallel,spaced-apart apertures (4), and wherein the identically configuredapertures (4) are formed for natural resonance for an interferencefrequency f (ind S1) to be blocked by the waveguide filter, and theapertures (3,4) are arranged in the conducting structure (2) such that,relative to a longitudinal expansion of the respective apertures, theapertures are positioned respectively in the center of a symmetry axisfor the conducting structure (2), which axis runs parallel to fieldstrength vectors (5) for the waveguide.
 2. A waveguide filter accordingto claim 1, wherein the waveguide (1) has a circular cross section.
 3. Awaveguide filter according to claim 1, wherein the centrally arrangedaperture (3) and the additional apertures (4) are designed as slottedguides, the slotted guides are aligned perpendicular to the vectors (5)of the adjoining waveguide-E-field, and the electrically effectivelength of the slotted guides is one half the wavelength of thetransmission frequency f (ind 0) or the interference frequency f (indS1), depending on the respective natural resonance frequency.
 4. Awaveguide filter according to claim 3, wherein the slotted guide widthsare tuned to the desired transmission and blocking effect.
 5. Awaveguide filter according to claim 1, wherein a second conductingstructure (2) is inserted in a cross-sectional plane of the waveguide,at a distance to the first conducting structure (2), with said distancecorresponding approximately to one fourth the wavelength of the selectedtransmission frequency f (ind 0), and the second conducting structure(2) comprises a central aperture (3) with a natural resonance at theselected transmission frequency f (ind 0) and two additional apertures(4), respectively with a natural resonance at a second interferencefrequency f (ind 2) to be blocked, which differs from the interferencefrequency f (ind S1) of the apertures of the first conducting structure.6. A waveguide filter according to claim 1, wherein the conductingstructure (2) is a metal foil with the apertures (3,4) formed therein.7. A waveguide filter according to claim 1, wherein the conductingstructure (2) is composed of a dielectric substrate that is metallizedon one side, and the apertures (3,4) are formed in the metal layer ofthe substrate.
 8. A waveguide filter according to claim 1, wherein thewaveguide (1) has a rectangular cross section.
 9. A waveguide filter,inserted as a first planar conducting structure in a cross-sectionalplane of an elongated waveguide, perpendicular to the waveguide axis,between a feeding and an extending segment of the waveguide, with thefirst planar conducting structure comprising at least one apertureconfigured for natural resonance for a selected transmission frequency f(ind 0), and wherein the conducting structure (2) has a centrallyarranged aperture (3), configured for natural resonance for aninterference frequency f (ind S1) to be blocked with the waveguidefilter; and, in addition to the centrally arranged aperture (3), the atleast one aperture of the first planar conducting structure (2)comprises two identically designed apertures (4), which are configuredfor natural resonance for a selected transmission frequency f (ind 0)and are arranged symmetrical to the central aperture (3).
 10. Awaveguide filter according to claim 9, wherein the centrally arrangedaperture (3) and the additional apertures (4) are designed as slottedguides, the slotted guides are aligned perpendicular to the vectors (5)of the adjoining waveguide-E-field, and the electrically effectivelength of the slotted guides is one half the wavelength of thetransmission frequency f (ind 0) or the interference frequency f (indS1), depending on the respective natural resonance frequency.
 11. Awaveguide filter according to claim 10, wherein the slotted guide widthsare tuned to the desired transmission and blocking effect.
 12. Awaveguide filter according to claim 9, wherein a second conductingstructure (2) is inserted in a cross-sectional plane of the waveguide,at a distance to the first conducting structure (2), with said distancecorresponding approximately to one fourth the wavelength of the selectedtransmission frequency f (ind 0), and the second conducting structure(2) comprises a central aperture (3) with a natural resonance at theselected transmission frequency f (ind 0) and two additional apertures(4), respectively with a natural resonance at a second interferencefrequency f (ind 2) to be blocked, which differs from the interferencefrequency f (ind S1) of the apertures of the first conducting structure.13. A waveguide filter according to claim 9, wherein the conductingstructure (2) is a metal foil with the apertures (3,4) formed in there.14. A waveguide filter according to claim 9, wherein the conductingstructure (2) is composed of a dielectric substrate that is metallizedon one side, and the apertures (3,4) are formed in the metal layer ofthe substrate.
 15. A waveguide filter according to claim 9, wherein thewaveguide (1) has a rectangular cross section.
 16. A waveguide filteraccording to claim 9, wherein the waveguide (1) has a circular crosssection.